Well, my fan died... and any app that measures temperature recommends closing the program you're running every time temp. reaches 98, so I don't know if that can be deemed as normal, to be honest.
Also, it was not happening before with the game.
The fact is, if you were to disregard the risk it poses to your computer (which you should't), the performance of the game is also affected (stuttering, glitches, etc.). That shouldn't be deemed as normal either.
It sounds like whatever app you are using is trying to pre-emptively warn you about something that shouldn't happen. The logicboard has a built-in safety measure that will actually force a shutdown if your internal temperatures reach unsafe levels to prevent hardware damage. This is called a "Thermal Shutdown" and will typically only happen if your heatsink and fans aren't seated properly or functioning. This means you have a computer that wasn't assembled properly or has faulty hardware in it. If you are indeed right about your fan being dead, then I wouldn't use the computer until I got that replaced. Stuttering and performance problems can arise from hardware that isn't performing properly, such as in the case of overheating. However, overheating is when you are reaching the threshold for the Thermal Shutdown to be triggered, not due to constant use.
For example, when I run handbrake to rip a DVD to my computer I can see temps up to 105C. This is because handbrake puts a LOT of demand on the hardware to do the video encoding as fast as possible. When playing demanding games, it isn't unusual to see temperatures of the CPU and GPU reach anywhere from 85-100C. Yes that is VERY hot, but that is well within the operating temperatures of the computer before you start reaching the "danger zone" for what is classified as "overheating". Like I said I would get that fan replaced, and if you are seeing the temps at levels you don't think are normal then put the computer somewhere that would get more airflow. It is also possible that you might have dust clogging your air-intake vents. To get those cleaned then take it to an applestore or a apple certified technical so that they can open it and clean it out for you.
You also have to give the temperature sense a bit of leeway on readings. They are pretty accurate, but always take it with a grain of salt considering it is a temperature sensor that is surrounded by metal that is retaining heat. always read the temperature it gives you as plus/minus a few degrees.