The phone heats up so much even while on call or charging ideally that when it cools down it gathers a lot of moisture/water inside the transparent case provided in the box. PLEASE PLEASE PLEASE fix this or the phone will get damaged for no reason and that will be ALL Nokia's fault
For moisture to appear inside the cover, then there must already be moisture in there - either the phone has been exposed to water or some water has got inside the cover. The phones cannot create water just by getting hot, the water must be put in there somehow.
I suggest that you take the phone out of the cover, put both in a warm dry place for several hours, put the phone back in the cover and try again. If you still get condensation inside the cover then this means that something you are doing is causing water to get inside, so check that all the places you keep the phone are waterproof.