We have employed Monte Carlo simulation in the NPT ensemble to determine the molar volume and the compressibility factor of naturally occurring hydrocarbon gas mixtures, The simulation results were compared to experimental data as well as to predictions based on both the Peng-Robinson and AGA 8 equations of state. The mean error of the simulation results was roughly of the same order as those of the equation of state values. For pressures higher than the retrograde dew point pressure, systematic deviations were found especially if the gases contained a high proportion of heavy components. The quality of the simulation results increased with rising temperature. In the region of retrograde condensation, we used the Gibbs ensemble technique to determine the molar volume as well as the composition of the phases. Both were in agreement with the results of flash calculations combined with the Peng-Robinson equation of state. At pressures far from the retrograde dew point pressure, the simulation results followed the experimental liquid drop out curve.