The total annual expenditure (TOTEX) is heavily determined by the cost of hydrogen and carbon dioxide supply with hydrogen being the dominant factor. Almost 97% of the ) is assumed. However, this leads to the heat balance for the reboiler not being maintained in all simulations. This simplification is justified by the fact that the cost of heat integration and the effect on CAPEX, OPEX and LCOM is fairly constant as demonstrated in and validation statistics are included in Table . As seen from the results, the surrogate models for LCOM, CAPEX, Power, OPEX, and methanol production shows good performance and fidelity for being used as a replacement of the full simulation model in optimisation.