New force fields for carbon dioxide and nitrogen are introduced that quantitatively reproduce the vapor - liquid equilibria (VLE) of the neat systems and their mixtures with alkanes. In addition to the usual VLE calculations for pure CO2 and N-2, calculations of the binary mixtures with propane were used in the force-field development to achieve a good balance between dispersive and electrostatic (qundrupole- quadrupole) interactions. The transfer ability of the force fields was then assessed from calculations of the VLE for the binary mixtures with n-hexane, the binary mixture of CO2/N-2, and the ternary mixture of CO2/N-2/propane. The VLE calculations were carried out using configurational-bias Monte Carlo simulations in either the grand canonical ensemble with histogram-reweighting or in the Gibbs ensemble.