A lot of devices (especially high performance ones) do heat up when running power-consuming apps. That's absolutely normal. However, if the heat bothers you, there's a number of things you can do:
1. Close down other applications before entering the game.
2. Make sure your device is at least 50% charged before you start playing.
3. Make sure your device is updated to the latest version.
If the heat affects device performance, we recommend contacting the manufacturer for further instructions.