A model is presented to evaluate the thermal-hydraulic performances of a combined-cycle plant HRSG. Horizontal HRSGs with economizers, superheaters and natural circulation evaporating sections, operating in a two-pressures cycle, are considered. Several finned staggered or in-line tube banks with spiral and circular fins arc considered in each section. The heat transfer and gas pressure drop in each tube bank and the steam flow rate in the evaporating sections are evaluated. Several thermal and geometrical parameters are also estimated to perform quick thermal design verifications and comparison between different steam generators sizings. An example of application is presented, by performing the comparison between four HRSGs with different sizings of a 340 MW_e combined cycle plant.