This study investigates the utility system of the pyrolysis unit of a polymer plant and assesses the decarbonization options of the utility system. The method is based on an integrated approach that includes a steady-state digital twin (SSDT) of investigated process and an update of the existing steam cascade to apply renewable energy and power-to-x. Pyrolysis gas composition was obtained from online analyzers and simulated in a UniSim environment. The simulation model of the whole process, from pyrolysis of liquid feedstock to final monomer separation, was developed and verified on plant data. The energy-saving potential was assessed based on SSDT, and energy-saving measures were proposed. The updated steam cascade of the investigated process was developed, and decarbonization options accounting for steam generation and waste heat utilization were proposed. The final solution gives the energy targets for e-boilers and low-grad heat for Organic Rankine Cycle (ORC).