Enthalpies for methane-ethane-carbon dioxide mixtures at low temperature and high pressure saturation conditions were calculated by the Gibbs ensemble Monte Carlo method. The Lennard-Jones potential function was employed to describe the site-site intermolecular interactions. Molecules of both methane and carbon dioxide were treated as spherical while those of ethane were treated as linear with two methyl groups. The total number of molecules adopted in all simulations was 300. A comparison of the simulated data with the results obtained from the Peng-Robinson equation of state yields a reasonable agreement.