In this study, an effort has been made to accurately correlate equilibrium interfacial tension (IFT) between methane and water/brine with their mutual solubility in a temperature range of 278.2-477.6 K and pressure range of 0.01-107.10 MPa, respectively, while comparisons have been made between the CO2-water pair and methane-water pair and then the IFT estimation is extended to ternary systems with a newly developed mixing rule. The newly developed model is found to reproduce IFTs between methane and water with an absolute average relative deviation (AARD) of 3.5% and greatly outperform the three existing correlations. Both a higher methane solubility in aqueous phase and a higher water solubility in hydrocarbon phase can decrease the IFT. Also, the new model can help explain the slight IFT change for methane-water pair but a simultaneously great change of their mutual solubility at low pressures. Temperature is found to exert an effect on IFT mainly through regulating the water solubility in hydrocarbon phase, while the pressure imposes an impact on IFT mainly through regulating the methane solubility in aqueous phase. The newly developed model can yield a reasonable estimate for the IFTs between natural gas and water/brine. It is found that using two sets of correlations and adding a pressure term are both necessary to account for the density effects on IFT for CO2-water pair in the vapor-liquid (aqueous) region at temperatures lower and near the CO2 critical temperature (i.e., 304.3 K), whereas they are not necessary for methane-water pair, whose density is changed more smoothly rather than sharply in the supercritical region. In addition, a new mixing rule is developed to successfully extend the IFT correlations based on mutual solubility to the ternary methane-CO2-water systems with an AARD of 5.2%. (C) 2018 Elsevier B.V. All rights reserved.