The aim of this study is to predict hydrate formation pressure and temperature through the three-phase (hydrate-liquid water-gas) equilibrium calculation using the GE-EOS and UNIQUAC activity coefficient models. Calculation of the hydrate formation conditions is carried out for pure gases such as CH4, C2H6, C3H8, iC(4)H(10), CO2, H2S and N-2, the gas binaries and ternaries; and some synthetic natural gas systems. Also calculations of the gas hydrate formation pressures are performed in the presence of methanol as inhibitor for the different gas hydrate systems. In this work, the Chen-Guo model is used for the hydrate phase and the UNIQUAC activity coefficient is applied for nonideality of the liquid phase. The SRK equation of state coupled with Huran-Vidal (HV) mixing rule is applied for the vapor phase. The UNIQUAC function is used not only for calculation of the activity coefficient of water in aqueous phase, but also is applied in the HV mixing rules. The UNIQUAC binary interaction parameters are adjusted through the hydrate experimental data directly so that there is no need to use the various vapor-liquid experimental data to obtain the parameters. To achieve More accurate results, the solubility of the gases in the aqueous phase is also taken into account using Henry's law. Finally, the present results are compared with the other available works and a very good agreement is observed with the experiment. The percent of Absolute Average Deviation (AAD%) for the calculated hydrate formation pressures and temperatures are considerably less than those results given by the other previous works. (c) 2014 Elsevier B.V. All rights reserved.