This study presents the optimization of a Heat Recovery Steam Generator (HRSG) system with integrated low-temperature heat recovery to enhance thermal efficiency and promote sustainable energy utilization in a combined cycle gas turbine (CCGT) power plant. The research addresses key industrial challenges, including high exhaust flue gas temperatures (~200°C), dependence on electric heating, and underutilization of waste heat. A secondary heat exchanger (Preheater-2) is introduced downstream of the Make-Up Water Heater (MUWH) to reduce the flue gas exit temperature to 60°C while recovering energy for potable water heating. The system was modeled and optimized using Aspen Plus. Results show that the proposed configuration can offset approximately 550 units of 3 kW electric heaters, resulting in a daily energy saving of 11.55 MWh. Thermodynamic performance improved, with HRSG heat duty increasing from 2.546 MW to 3.711 MW and overall thermal efficiency rising from 61.25% to 64.76%. Sensitivity analysis identified an optimal potable water flow rate range of 60,000–70,000 kg/h, yielding stable outlet temperatures of about 80°C. Exergy analysis confirmed reduced system irreversibility. The low sulphur content of Nigerian natural gas supports safe low-temperature heat recovery without corrosion risk. The system offers a scalable solution for industrial waste heat recovery, with applications in process heating, domestic hot water generation, and energy cost reduction.