I've not seen pictures of the internals, but the main SoC (CPU, GPU, modems, radios, etc) is usually close to the camera. The Snapdragon 835 is a high performance, 8 core unit; capable of generating significant heat. It's quite normal to get some warmth there with even moderate usage. Under heavy usage, it can get quite hot. These SoCs can generate far more heat than can be removed by the passive cooling available (with the expectation that they will not run at 100% for prolonged periods). There is thermal protection which automatically cuts the performance when the chip reaches max safe operating temperature.