We have used the Gibbs ensemble Monte Carlo (GEMC) method to calculate the vapor-liquid equilibrium properties for pure propane and methanethiol as well as their mixtures. We used two different potential-energy surfaces to describe the propane and methanethiol molecules. The results show that the simple site-site potential-energy surfaces work well for the pure components by optimizing the parameters to reproduce the liquid-phase density at the normal boiling point. For mixtures, the site-site potential-energy surface predicts the liquid-phase densities fairly well, but is not accurate enough to give quantitative results for the vapor-phase mole fraction for this system.