•Innovative method is used to study mass transfer phenomena of direct-contact condensation.•Nonequilibrium liquid–vapor surface is constructed with temperature/pressure drop.•Non-negligible anisotropic region exists by liquid–vapor interface in preliminary stage.•Evaporation and condensation coefficients are not equal.•Modified Schrage relation shows better consistency with the MD results. Direct-contact condensation plays an important role in both natural and engineering sciences. Nonequilibrium molecular dynamics (MD) simulations were conducted in this study to examine the mass transfer during transient direct-contact condensation. We designed an innovative method to construct a nonequilibrium liquid–vapor surface with sudden temperature and pressure drop between the two adjacent sides of the interface. This state was consistent with the initial state of real direct-contact condensation and verified the effectiveness of this method. The MD results indicated that a non-negligible anisotropic region existed near the liquid–vapor interface in the preliminary stage of direct-contact condensation when the nonequilibrium and transient phenomena were obvious. The velocity distributions of the vapor were time- and space-dependent, and the condensation and evaporation coefficients were not equal. Validation of the Schrage relation was conducted to calculate the net condensation rate. The results indicated that the Schrage relation modified by the temperature anisotropy and the evaporation coefficient shows better consistency with the MD results. For engineering applications, the time- and space-dependencies of velocity distributions are regarded having minimal influence on the validity of modified Schrage relation.