Getting warm is very much a normal thing when playing games or using CPU intensive apps. However if it is getting warm enough that it becomes uncomfortable, that is a different thing entirely. Anything that works hard like that will get warm, be it a phone, computer, mechanical equipment, etc. They all have their heat limits on what is safe and what isnt. With a phone that limit is when it gets uncomfortable to hold in your hands. IF it starts doing that, get it looked at.