The Green-Kubo formalism for evaluating transport coefficients by molecular dynamics has been applied to multicomponent mixtures of flexible, multicenter models of linear and branched alkanes and nitrogen and helium in the gas phase and in the liquid phase. Simulation results on binary systems are summarized and trends in prediction using simple but realistic molecular models are shown. New simulation results of N-2-n-pentane agree with experiment with a maximum deviation of 36%, the greatest error being for pure n-pentane. Methodological aspects of simulating multicomponent systems with trace components are studied, varying the system size and molecular interaction potentials. It is shown that mixtures are treated representatively even when only one to two molecules of a species are present in the simulated system, unless there is an extreme degree of self-association. It is demonstrated that molecular dynamics may predict quantitatively (7% and 11% deviation) the viscosity of a seven component "synthetic" natural gas.