In this paper, feasibility of installation of duct burner in an unfired Heat Recovery Steam Generator has been investigated. According to the primary design, the proposed HRSG doesn't consist of supplementary firing consideration, so in this study thermal resistance of the heating surfaces is investigated at first and then the cooling system is investigated whether it has an appropriate capacity to release the additional heat from the system or not. In addition, a detailed economic analysis has been conducted to have better insight into the models, also the social cost of CO2 emission has been considered. Thermodynamic relations have been used to model the power plant. The results show that using duct burners, increases the power output but it leads to decrease in power plant efficiency. Although to keep the condenser vacuum in a reliable range, cooling system should work with its maximum rate.