This paper discusses the known methods for calculating the efficiency coefficient of an adiabatic compressor when compressing single-phase and two-phase mixtures of hydrocarbons in a wide range of temperatures and pressures. It is demonstrated that the applicability of these methods is limited relative to real gases, including two-phase gas mixtures with condensate particles. For real gases, the range of possible variations of the polytropic efficiency coefficient has been determined. New method for calculating the polytropic efficiency coefficient as the half-sum of the values of the isoentropic and exergy efficiency coefficients, with a correction which value is determined in accordance with the basic principles of thermodynamics has been proposed. The proposed method does not require assumptions on the properties of real gas and on the method of defining the polytropic process. The calculated data coincide, with sufficient accuracy, with the experimental data obtained in testing natural gas-compressing compressors at the factory testing stand at the production enterprise of Sumy Maсhine-Building Science and Production Association.