A method of controlling a heat recovery steam generator (HRSG) includes measuring a first regulated output of the HRSG and a second regulated output of the HRSG. The method includes comparing the first regulated output to a first setpoint defining a first target output to generate a first error signal and comparing the second regulated output to a second setpoint defining a second target output to generate a second error signal. The method also includes generating, by a controller implementing a multivariable control algorithm having as inputs the first error signal and the second error signal, control signals to control the HRSG to adjust values of the first regulated output and the second regulated output.