Here we present a predictive method for the change in the three-phase (vapor-liquid-hydrate) equilibrium condition of gas hydrates upon the introduction of organic inhibitors and electrolytes. The Peng-Robinson-Stryjek-Vera (PRSV) equation of state, combined with the COSMO-SAC activity coefficient liquid model through the modified Huron-Vidal (MHV1) mixing rule, is used to describe the fluid phase, and the van der Waals and Platteeuw (vdW-P) model is used to describe the hydrate crystalline phase. The temperature-dependent Langmuir absorption constants for the vdW-P model are determined by fitting to the equilibrium condition of pure gas hydrates. Once determined, the method contains no adjustable binary interaction parameters and can be used for prediction of the phase behaviors of gas hydrates with additives that do not enter the cages of the clathrate hydrates (e.g., most inhibitors and electrolytes). We examined the accuracy of this method using five pure gas hydrates, five organic inhibitors, and nine electrolytes, and over ranges of temperature (259.0-303.6 K) and pressure (1.37 x 10(5)-2.08 x 10(8) Pa). The average relative deviations in the predicted equilibrium temperatures are found to be 0.23% for pure gas hydrates, 0.72% with organic inhibitors, and 0.18% with electrolytes, respectively. We believe that this method is useful for many gas hydrate related engineering problems such as the screening of inhibitors for gas hydrates in flow assurance.