In addition to new binary interaction parameters (BIPs) which are regressed with mutual solubility data of binary mixtures, Peng-Robinson equation of state (PR EOS) together with two newly modified alpha functions respectively for non-water components and water are used to determine the two-phase compositions for methane-CO2-water mixtures and three-phase compositions and three-phase upper critical end points (UCEPs) (i.e., the pressure at which the alkane-rich phase becomes identical to the CO2-rich phase at a given temperature) for propane/n-heptane/n-decane-CO2-water mixtures (i.e., Model #1) at temperatures and pressures up to 423 K and 14500 psia (100 MPa). Model #1 is found to accurately predict the two-phase compositions for methane-CO2-water mixtures. Model #1 yields much better predictions of three-phase compositions for propane/n-heptane/n-decane-CO2-water mixtures than the existing SAFT-VR (i.e., statistical associating fluid theory-variable range) model (i.e., Model #2), whereas the prediction accuracy with both Models #1 and 2 for x(W)(non-aq1) (i.e., the molar fraction of water in alkane-rich phase) and is still compromised. By comparing the molar fractions between binary mixtures and ternary mixtures, it is found that the addition of CO2 can significantly reduce the methane solubility, but enhance the propane/n-heptane/n-decane solubility in aqueous phase (i.e., water-rich phase), while addition of methane can greatly reduce the CO2 dissolution in aqueous phase. Such an effect for methane-CO2-water mixtures is closely related to the molar fraction of methane (i.e., the ratio of the molar fraction of methane to that of methane + CO2 in feed). The addition of CO2 can significantly enhance the water dissolution in alkane-rich phase. Model #1 yields accurate predictions for three-phase UCEPs and critical phase compositions except for the water molar fraction. Overall, for the predicted three-phase UCEPs, BIPHC-CO2-cot imposes the largest effect, BIPHC-Wnon-aq ranks the second, and BIPCO2-Wnon-aq, takes the third place. BIPHC-Waq and BIPCO2-Waq exert no effects on the predicted three-phase UCEPs.