The gas-cooler heat exchanger is a characteristic device of the refrigeration plants that use CO2 as the working fluid in transcritical conditions. The proper gas-cooler sizing, allows getting the maximum effectiveness in the heat exchange, thus permitting to reduce the temperature approach and improving the heat recovered in the coolant (secondary) fluid and the COP of this kind of systems. This work deals with the development of a steady-state coaxial gas-cooler model that uses water as coolant fluid. The model is based on the finite-volume methodology and is validated with experimental data. An uncertainty lower than +/-10% was obtained in the calculation of the heat transfer rates and an uncertainty lower than +/-3 degrees C in the outlet temperatures of both fluids. Several correlations for estimating the CO2 heat convection coefficient have been studied to select the one that best fits the model. Using the model, the influence of different parameters on the gas-cooler thermal effectiveness has been analyzed and presented in this work. Finally, a comparison between the traditional E-NTU method and the developed model has been done. (C) 2012 Elsevier Ltd. All rights reserved.