• A novel flue-gas cooling-reheating system for coal-fired power plants is proposed. • The thermodynamic system is optimised under fixed operating parameters. • The rank sum ratio method is applied to working-fluid selection for the first time. • The trade-off among environment, health, safety, and costs is assessed. An upgraded flue-gas cooling–reheating system called an organic gas–gas heater, which builds a heat-pump cycle downstream of the wet desulfurisation process, is proposed for coal-fired power plants. Compared with the existing system, it can improve the particulate matter 2.5 emissions and alleviate sulfuric-acid corrosion. Adjustability of operating parameters is restricted by the standards in power industry and the automatic control habit of users. In view of this, multi-objective optimisation was conducted for the organic gas–gas heater system, on the premise of unalterable heat-source and heat-sink parameters and fixed working-fluid evaporating and condensing temperatures. According to comparison results for 31 selected working fluids under six operating conditions, it is feasible to realise a trade-off among thermodynamic, economic, and environmental objectives, even though the sole alterable parameter is the type of working fluid. The rank sum ratio method was applied to the field of working-fluid selection for the first time. As a comprehensive assessment method, it was confirmed in this study to be simple and effective. Moreover, R245ca is the optimum working fluid in this study, but the results indicate that the priority of the working fluid must be reconsidered if the operating conditions or assessment criteria change.