In this paper, the mean-field lattice-gas model by Koningsveld and Kleintjens is applied to describe vapor-liquid equilibrium. For petroleum or petroleum fractions, one has to consider additionally the multicomponent character of the system. To handle the problems with the large number of similar components, continuous thermodynamics has been often proved to be a convenient method. Here, the composition of the multicomponent mixture is described by continuous distribution functions. On this basis, for systems consisting of a petroleum fraction and a polar component, equilibrium-flash-vaporization curves at low pressures are predicted. The parameters needed were fitted to phase equilibrium data of pure substances and binary mixtures. There is a reasonable agreement between calculated and experimental results.