A waste heat recovery plant control system includes a programmable controller configured to generate expander speed control signals, expander inlet guide vane pitch control signals, fan speed control signals, pump speed control signals, and valve position control signals in response to an algorithmic optimization software to substantially maximize power output or efficiency of a waste heat recovery plant based on organic Rankine cycles, during mismatching temperature levels of external heat source(s), during changing heat loads coming from the heat sources, and during changing ambient conditions and working fluid properties. The waste heat recovery plant control system substantially maximizes power output or efficiency of the waste heat recovery plant during changing/mismatching heat loads coming from the external heat source(s) such as the changing amount of heat coming along with engine jacket water and its corresponding exhaust in response to changing engine power.