Simulations are presented of the liquid-phase-vapor-phase equilibria of binary mixtures (methane + ethane, methane + carbon dioxide, ethane + carbon dioxide) and ternary mixtures (methane + ethane + carbon dioxide) in the Gibbs ensemble. Methane, ethane, and carbon dioxide are described with molecular models of the types 1CLJ, 2CLJ, and 2CLJQ, respectively. The equality of the chemical potentials in the two phases is maintained by transfer of the smallest molecule and exchange for the remaining molecules. The successful acceptance ratio of exchange is usually more than 10 times larger than that for transfer. The results are in relatively good agreement with experimental and other simulation data.