The goal of surfactant-polymer flooding (SP) is to reduce interfacial tension (IFT) between oil and water so that residual oil is mobilized and high recovery is achieved. The optimum salinity and solubilization ratio that correspond to ultra-low IFT have recently been shown to be a strong function of the methane mole fraction in the oil at reservoir pressure in some cases. We incorporate a recently developed methodology to determine the optimum salinity and solubilization ratios at reservoir pressure into a chemical flooding simulator (UTCHEM). The proposed method determines the optimum conditions based on density estimates using a cubic equation-of-state and measured phase behavior data at atmospheric pressure. The microemulsion phase behavior (Winsor I, II, and III) are adjusted based on these predicted optimum salinity and solubilization ratio in the simulator. Parameters for surfactant phase behavior equation are modified to account for these changes and the trend in equivalent alkane carbon number is automatically adjusted for pressure and methane content in each simulation gridblock. We use phase behavior data from several potential SP floods to demonstrate the new implementation. The implementation of the new phase behavior model into a chemical flooding simulator can greatly aid in the design of SP floods so that SP flooding failures are less likely to occur. The simulator will also make more accurate estimations of oil recovery. The new approach could also be used to handle free gas that may form in the reservoir. We show that not accounting for the phase behavior changes that occur when methane is present at reservoir pressure can greatly affect the oil recovery of SP floods. Improper design of a SP flood can lead to more oil being produced as a microemulsion phase than as an oil bank. This paper describes the procedure to implement the effect of pressure and solution gas on microemulsion phase behavior in a chemical flooding simulator, which requires the phase behavior data measured at atmospheric pressure.