A reliable prediction of the phase behavior is necessary in determining the compositions of the gas and liquid phases at various pressures. These calculations require stepwise computational procedure using a cubic equation of state (EOS). Since the heavy components in the petroleum mixtures have the strongest effect on the characteristics of the fluids, critical properties must be estimated for the petroleum fractions making up heptanes-plus. A phase equilibria calculation of a gas-condensate system with Peng-Robinson equation of state was done by satisfying the condition of chemical equilibrium. A genetic algorithm was used to determine the optimum critical properties of heptanes-plus (C7+) fraction. The predictions of the model are compared with the experimental results of the constant volume depletion (CVD) test.