This is something that I’ve seen a LOT. And it’s rarely a problem, even though some tries to make you think that.
Mobile phones warm up when they’re working hard, or when they’re transferring a lot of data via the network. This is just how it works. It’s normal. On a HTC phone, when the temperature gets worryingly high, it flashes the notification LED, makes a sound, and warns the user. When the temperature gets close to “Unsafe to operate”, it shuts down.
Any temperature which is below “flashing red and green” is normal, safe and fine. Yes the phone gets warm in your hand, but this is not a problem. Even when it IS flashing red and green – this too isn’t a problem but you should maybe think about stopping your game or doing something less intensive for a while.
A phone that doesn’t get a little warm, that would make me worried since it’s supposed to get a little warm. It’s simply how a processor works.
AND: it depends on surrounding temperature of course. If you sit in a room that’s 36°C, then of course your phone WILL get warmer. It’s basic common sense.
Sure: phones does overheat and is different from a phone getting a little warm, but it’s not as common as some wants to make you think. I’ve never seen it once myself, but I live in Sweden where it perhaps gets over 30°C once or twice a year.