A new Rankine power cycle utilising a combination of ocean thermal energy and geothermal waste energy is proposed and thus called a GeOTEC (Geo-Ocean Thermal Energy Conversion) power cycle/plant. The potential geothermal waste heat, which exists in the form of raw hot natural gas is continuously pumped from a shallow water Malaysia-Thailand Joint Authority (MTJA) gas production platform, and the supply data is estimated based on the output of the platform. A thermodynamic model derived from an energy balance calculation is used to simulate the proposed GeOTEC cycle with Matlab. A capital cost estimation is performed for the proposed GeOTEC based on the commercially available components and manufacturing practices. With higher superheated ammonia temperature, GeOTEC power plant efficiency increases, while the net power output decreases. A maximum net power produced by the proposed GeOTEC is 32.593 MW with estimated capital cost of USD4,489/kW.