Abstract The article is considering a recent mathematical method of phase equilibria calculation of natural gas condensate systems taking into account the presence of brine in reservoir media. This method is based on a three-parameter Peng - Robinson cubic equation of state and Huron - Vidal mixing rule. The applicability of the PVT-modeling method is illustrated by comparison of the calculation results and experimental data by the examples of binary mixtures of water with methane, ethane, propane, n-butane, nitrogen, carbon dioxide and hydrogen sulfide. The new values of Huron - Vidal mixing rule parameters are proposed, allowing to improve the accuracy of phase equilibria calculation for binary mixtures of natural gas condensate systems component and water in a wide range of pressure and temperature. A comparison of the results of engineering formulas (Bukacek method) based and a mathematical method (equation of state and Huron – Vidal mixing rule) based calculations of hydrocarbon mixture water content was made. The effect of the residual water presence on the gas condensate system phase behavior at reservoir development is evaluated on the base of constant volume depletion (CVD) test mathematical modeling with the application of the discussed method of phase equilibria calculation. Examples of residual water presence effect on the PVT-properties of natural gas condensate systems distinguished by initial reservoir conditions, potential content of stable condensate (C5+) and composition are considered.