A new simplified fully implicit compositional simulator has been developed. This simulator is different from existing models because it combines the conventional black oil equations with a compositional injection component equation. The simulator runs at speeds that far exceed the speeds in current compositional simulators, primarily because the speed is independent of the number of components. Data preparation is both simplified and improved because component pseudoization is not needed. The simulator has been tested successfully for gas cycling projects including the Third SPE Comparative Solution Project and CO2 flooding projects.