This paper presents the design of a waste heat recovery system for a China VI emission standards (China VI) natural gas engine to address cooling challenges. A heat balance bench experiment was conducted to characterize the waste heat properties of the engine's exhaust, exhaust gas recirculation exhaust (EGR), and coolant. A transcritical dual-loop Rankine cycle system was modeled and analyzed using MATLAB to recover this waste heat. The system utilizes two loops: high-temperature loop to recover exhaust waste heat and EGR waste heat and low-temperature loop to recover waste heat from the coolant and high-temperature loop, improving overall recovery. The low-temperature loop is based on a transcritical cycle, making the system more compact than a subcritical alternative and better suited for vehicle applications. Analysis identified water as the working fluid for high-temperature loop and R32 as the optimal working fluid for low-temperature loop. The Rankine cycle waste heat recovery system was found to increase the engine's power output by approximately 71.29 kW at rated operating conditions, representing a 21.03% gain over the original power.