In this paper, a combined steam-organic Rankine cycle is proposed to recover the waste heat of a gas turbine. Proposed combined system includes a subcritical steam Rankine cycle that is coupled with a transcritical organic Rankine cycle. Three different organic fluids such as R124, R152a, and R134a are selected to monitor the thermodynamic and exergo-economic performance of the system. Results show that maximum exergy efficiency and minimum total product cost rate of a studied system for the base case condition are 57.62% and 396.7 ($/h) for the combined cycle with R124 and R152a, respectively. Also, a parametric study is performed to investigate the effects of key parameters including steam turbine inlet pressure, organic turbine inlet pressure, organic preheater pinch temperature and organic condensation temperature on exergetic efficiency and total product cost rate of the system. Finally, the Genetic algorithm is employed to conduct a multi-objective optimization of the system with two objective functions including exergy efficiency and total product cost rate. The results of optimization revealed that combined cycle with R152a has the best performance from thermodynamic and exergo-economic viewpoint among analyzed fluids. (C) 2016 Elsevier Ltd. All rights reserved.