Abstract: The article describes the effect of phase transition heat on the temperature of a closed multicomponent vapor–gas metastable phase and on growing supercritical droplets and their size distribution at the stage of homogeneous formation and growth (nucleation stage) of the supercritical droplets. It is assumed that, between essentially supercritical droplets and the vapor–gas medium, a stationary diffusion transfer of condensing vapor molecules and heat is established, and, then, both the composition and temperature remain unchanged and the same for all supercritical droplets. A set of equations is derived to determine the composition, temperature, and growth rate of the essentially supercritical droplets via the initial temperature and supersaturation of vapors. Expressions are obtained for the deviation of vapor–gas medium temperature from its initial value and for the droplet size distribution function as depending on time.