It is shown that a multicomponent two-phase system moving through a porous medium may be described by a limit model in which the thermodynamic subsystem is totally separated from the hydrodynamics. The limit corresponds to a contrast phase mobility and a fast pressure relaxation process. The obtained limit thermodynamic model includes several differential thermodynamic equations and describes the equilibrium in an open system. The model is validated by comparing with the full compositional flow simulations. A numerical solution to the limit thermodynamic model is constructed. An application to the gas-condensate systems is compared to the full compositional model.