Laptop heating up when put it in bag

Solution 1:

I think Suspending your system keeps parts on, like you said. That is the default for Ubuntu. Windows, on the other hand, uses Hibernate by default. Try using Hibernate instead

How to enable hibernation?

Solution 2:

It would be better if you find out whether SLEEP (which is called as SUSPEND in Ubuntu) really works for you. If sleep isn't working properly, then it's as good as only turning off the screen and keeping all other things running.

So see if comparable heating occurs when you put your laptop to SUSPEND and leave it on the desk.

If SUSPEND not working is the reason, then you might have to work on getting that working, which could automatically fix your present issue.