A stack system and method for in-line geothermal and hydroelectric generation from recovered natural gas-fired water/steam process waste heat. The stack system and method is a new way of reusing natural gas fired water/steam process waste heat to make more electricity from the same Btu inputs. The stack system and method uses warm industrial demineralized water from various sources, a micro-managed combined stack flue system and specific terrain to ring out every bit of energy possible from traditional, heretofore, acceptable wastes. The stack uses two marginal waste heat sources to make one significant heat source for additional fossil fuel-free generation. This stack is unique in that it incorporates tandem, geothermal and hydroelectric generators. The stack can be applied to closed-loop (Power Stack) and open-loop (Desalination Stack) processes.