Although there are many simulation studies of mixtures, there remains a gap in the understanding of the phase transitions occurring when molecules are progressively added to a closed system. When molecules are added, the fluid passes from a rarefied state through metastable states where liquid droplets are dispersed in a gas phase. The droplets increase in size and when a sufficient number of molecules is present in the system, the droplets coalesce and the system splits into vapour and liquid phases separated by a planar interface. Here we have explored the microscopic details of these phase transitions for a methane/xenon mixture at 189 K by performing canonical ensemble simulations using the kinetic Monte Carlo scheme for uniform and non-uniform systems. When vapour and liquid are in equilibrium, the interface separating the vapour and liquid phases is planar. We have investigated variations in pressure and composition, in the interfacial region, when mixtures of molecules of constant compositions are added into the system. These changes are different from those in single-component systems because the increase in the number of particles affects the pressure of the system and the compositions of the liquid and gas phases.