IEM Kolkata is a very prestigious private college in West Bengal and it is basically known for some of its branches such as Computer science, Information Technology and Electronics, and Communication engineering. 

Mechanical at IEM:

• Few new branches including Mechanical introduced in the institution that has been added in the recent year.
• The students do not have much of a college life. There are not any extracurricular activities.
• There are only a handful of supportive faculties in these branches.
• There are no proper classrooms, labs for the mechanical branch. The Mechanical engineering department is situated on the Management campus.
• The students are made to follow some irrelevant rules and attend classes and seminars that have nothing to do with their future goals.
• The course curriculum is not industry-oriented. 
• In placements, students are mostly placed in IT companies.
• You won’t get much assistance for GATE preparation here too. Faculties are not very supportive and focus more on attendance.
• Although, students of the Mechanical branch at IEM are doing a good job in student chapters like IEEE, ISHARE, etc. 

The college is good for other branches but for mechanical, you should look for other options.

In terms of college, IEM and HIT both provide similar career opportunities in the IT sector. Major recruiters like TCS, Infosys, CTS, Capgemini from the IT sector visit both the institutes. 

If you had to pick between the two engineering fields, you should always go with CS. If you select CS, you will be able to apply for a variety of government/public sector positions (ISRO, BARC, IOCL) after completing your B.tech. Only CS guys are qualified to apply for the majority of these government jobs. 

As you will get similar opportunities you can choose either. Also, outside of WB, no one cares if you're from IEM or HIT. Unless you're from JU, everything is the same.

When choosing an engineering college, it is always somewhat better to have your options, but at the same time, stick to government-run institutes. That clarifies most of the issues here. NIT Silchar and NIT Tripura are both prominent institutes to choose from. 

Few others, unlike them, are mostly on the higher lists of popularity because they have their name inculcated as NIT. However, you should never feel left out if in any case you do not get acknowledged on grounds of NIT. The other best possible option, just second to NIT, is KGEC. 

Although, there is a short list of people who actually end up recommending any private institutes in lands of West Bengal.

If you anyhow need to get enrolled in any of the private institutions, then going to institutes that are deemed well known, such as SRM, KIIT, Manipal can be some other good options from your pocket.

If not any of them, then choose going to COMEDK or try enrolling in other renowned private colleges in Bangalore, some of which are RV College of Engineering, PES Institute of Technology, BMS College of Engineering and MS Ramaiah Institute of Technology.

There is another information yet to share which is, unlike BITS-Pilani, BIT-Mesra and MIT-Manipal,IEM is known to admit various students through management quota which is rather not the case for many other prestigious private engineering institutes.

But, as mentioned earlier, getting to choose from a handful of governmental institutes are better than the latter. However, options other than those few are opened as well.

As I know some people from these institutes you mentioned above, it will be clearer to base their ideas upon the rankings on different factors. 

In terms of Academic Credentials and Research works:

• Kalyani Government Engineering College will be my first choice. It is a Government-run College bestowed with some great Professors.
• Heritage Institute of Technology will be 2nd, as it also has some of the most influential professors. It is an autonomous institute. 
• This will be followed by Jalpaiguri Government Engineering College.
• Finally, the Institute of Engineering and Management, as there have been reports of faculty shortage.

In terms of Social life and Co-curricular Activities:

• Heritage Institute of Technology tops this criteria. 
• Whereas, Kalyani Government Engineering College is in the 2nd position.
• Followed by Jalpaiguri Government Engineering College and IEM.

In terms of Locational merits and Infrastructure,

• Heritage has one of the best infrastructures and is apparently only 10 mins from the major junction of Ruby Hospital or Desun Square.
• IEM also has established both the criteria really well, yet it somehow gets difficult for South Kolkata students.
• They are followed by Kalyani Government Engineering College and Jalpaiguri Government Engineering College.

In terms of Placement Scenario,

• Institute of Engineering and Management and Heritage Institute of Technology are the same in terms of their placement drives.
• Followed by Kalyani Government Engineering College and Jalpaiguri Government Engineering College

In terms of Affordability: (Fees, Accommodation, Miscellaneous Expenses)

• Kalyani Government Engineering College
• Jalpaiguri Government Engineering College
• Institute of Engineering and Management and finally,
• Heritage Institute of Technology

Hope this layer to layer breakdown has helped you gain some insight about the question you asked. 

The placement conditions of IEM are better than other private engineering colleges of West Bengal. There is no such procedure for getting placements in IEM. Overall, IEM gives a good average placement. Around 97% of students get placed. However, it entirely depends on the company's requirements. 

Some of the basic things which are considered during placements are as follows: 

• Academic score: Make sure your aggregate marks are more than or equal to 60%. 
• Gap year: You should not have any gap year.
• Communication skills: Work on your communication skills and read English newspapers to improve your vocabulary. 
• Aptitude: Practice Verbal and Non-Verbal aptitude, Logical Reasoning, and Quantitative aptitude thoroughly. Refer to RS Aggarwal to practice these questions. 

Have a positive attitude and you will crack it. 

IEM Kolkata has 4 core branches. The branches are as follows. 

• Electrical Engineering (EE) 
• Electrical and Electronics Engineering (ECE) 
• Mechanical Engineering (ME) 
• Electronics and Communication Engineering (ECE) 

The Department of Electrical Engineering at IEM is making efforts to produce trained, and capable technocrats who can take up the challenges of the real world. This department is doing pretty well in terms of placement and academics. 

The Electrical & Electronics Engineering Department is the newest and one of the best engineering departments of IEM. It is affiliated to the Maulana Abul Kalam Azad University of Technology. So, if you are considering IEM Kolkata to pursue ME, then without any second thoughts, just go for it.