A power generation system having a combustion engine with a Rankine bottoming cycle, the system including a first flow path for a process fluid and a second flow path for a working fluid, and a heat exchanger arranged along both the first and the second flow paths to transfer waste heat from the process fluid to the working fluid. The heat exchanger includes a first flow conduit being bounded by a first wall section and configured to convey the process fluid, a second flow conduit to convey the working fluid, the second flow conduit being bounded by a second wall section spaced apart from the first wall section to define a gap therebetween, and a thermally conductive structure arranged within the gap and joined to the first and second wall sections to transfer heat therebetween, the gap being fluidly isolated from both the process fluid and the working fluid.