A plant (1) and a gas processing unit (GPU) (17) of the plant can be configured to operate in accordance with a method that is configured to permit the GPU (17) to operate such that the optimum operating point for the GPU (17) at steady state to produce liquid carbon dioxide product from a separation unit (117) of the GPU (17) for sending to a storage device (19) is achieved with a desired purity level while simultaneously maintaining a required minimum carbon capture rate with the minimum consumption of power and/or minimum economic cost associated with operations of the GPU (17). A controller (23) can be configured to communicate with elements of the GPU (17) to receive parameter values to calculate manipulated variables configured to bias set points for parameters used to control operations of different elements of the GPU (17).