Actually, I do know a lot about refrigeration. None of the guesses by other posters can adequately account for the high heat in the mullion area as well as elsewhere on the outside of the cabinet. For example, a problem with the defrost heater is too far away from the mullion to heat it up.

The condenser coils run through the mullion area and the sides of the refrigerator. If there is a problem with the condenser fan, or if the condenser is blocked with lint, the heat from the condenser will not be blown off and it will travel to the mullion, where the OP sensed it. Fixing the fan or removing lint will solve the problem.