Basically, even small games like Rogue Legacy, Battle World: Kronos, Magicka, Crusader Kings, etc. These are not graphic intensive games, yet the laptop reaches temperatures over 90 C. However, this doesn't happen with Hearthstone. So, out of curiosity, why does this happen? Will it happen with a current-gen iMac?
Although the games may not be graphically intensive, they may still be intensive on the CPU, and it is likely the CPU temperature you are seeing rise.
A way to check this would be to use Activity Monitor.app while playing a game to check out the CPU usage. Or if you are comfortable and able, SSH into the Mac while playing the game and using the
top command to check.
The CPU chip is the brain of the computer.
Explained in most simple terms, it works with 1 and 0. Like the light switch on and off.
In order for it to create a single number or letter it needs 8 switches working together.
It does the switching millions of times per second, with millions of switches at the same time. The more it has to do the more switches are switching and faster.
The switching requires energy, thus the more it has to work the more energy it consumes. For example if you turn on the light switch once you do not feel like consuming any energy. Now try doing that in repeated motion (as fast as you can) for long time with both hands. You will feel the energy consumption, and eventually end up in sweet. Same with the CPU, except it switches millions of switches at the same time.
The CPU is also encapsulated in protective coating, like you wearing a winter coat, that would add to heating up.
To help CPU with staying cool, there is a big aluminum block on top of it. There is also a fan blowing air to cool it.
Bottom line the more the CPU has to do the more it will heat up.
The more powerful is the CPU the less it will heat up compared to its predecessor for the same task. A cruel comparison, if you were to force a CPU from 10 Years ago to play any of the "Rogue Legacy, Battle World: Kronos, Magicka, Crusader Kings" it would probably go red hot and melt.
As for the GPU (Graphics), same rules apply. In most Apple computers GPU function is a integrated part of the CPU chip, but some have a extra chip (called dual graphics) for more intensive graphics usage.