The process of water vapor condensation in a rocket engine exhaust in the upper atmosphere is considered. Effects of a heating of condensed particles by the latent heat of condensation, radiating heating and energy losses due to radiation are taken into account. The formed condensate is not in heat equilibrium with gaseous combustion products, and a dominating process responsible for energy losses of condensate is their thermal radiation. By solving the equations of heat and mass balance of the condensed particles, dependences of changes of temperature and size of particles upon time are obtained. During a condensation process, the thickness of condensed layer on the surface of particles can reach more than 70 A.