The coke oven plant has a central role in the iron and steel making process in an integrated steel plant. The subject of this research is to study how the production and energy system at the steel industry, with a connected combined heat and power plant, is affected by renovation of the coke oven. The aim is to investigate the interaction between the different processes and how the choice of system boundary affects the operation practice for the steel plant. MILP-based optimization models have been developed and used for the evaluation. The analysis shows that it is very important to take the interactions between the different production units in the system into consideration when making the analysis. A system optimization with a boundary including the whole system has a greater potential for minimizing the total system cost than one that only includes the processes where the actual changes are made. Conclusions are also drawn regarding the production practice for the specific system