In this paper, a comprehensive analysis of a combined Rankine cycle (water) and an organic Rankine cycle (two-stage organic Rankine cycle) is presented, in which the heat of the low-temperature flue gas of a 123.5 MW combined cycle power plant unit together with energy Liquid natural gas refrigerant can be effectively recovered and used. A two-stage organic Rankine cycle is proposed to recover the waste heat of the low temperature exhaust gas from the chimney of a combined cycle power plant. The proposed combined cycle produces output power and vaporizes liquefied gas simultaneously. The aim of this research is parametric analysis and thermoeconomic optimization of the proposed system. Based on thermodynamic mathematical models, the effects of key thermodynamic design parameters on system performance are tested from both thermodynamic and economic perspectives. The proposed two-stage system shows a significant output power of 49.6 MW from the Rankine cycle and 3.8 MW from the organic Rankine cycle. A detailed exergy analysis shows that the condenser is the main site of exergy destruction, accounting for 3254 kW in the Rankine cycle and 714 kW in the organic Rankine cycle, raising concerns about operating costs of $0.8 and $0.25, respectively. The hour is estimated. The highest recorded exergy efficiency is 86% for the Rankine cycle superheater and 85% for the organic Rankine cycle reheater, contributing to an overall cycle efficiency of approximately 80% and an exergy efficiency of 72%.