No, it isn't normal for any phone to get extremely hot. I don't know the cause, but I think that repairer attention is indicated.
I'll ask around a little.
Edit later: the S6 may be no stranger to heat, it would seem.
I'll request a thread move to the S6 forum for more comments.
Same problem here on my S6 Duos. The part of the phone where the power button and SIM tray are is nearest to the processor, and you should avoid using Mobile Data too much and check for rogue apps straining the RAM and CPU. It solved the issue for me.