This work analyzes the impacts of deploying a Power-to-Gas technology in the power generation sector in South Korea by 2050. The Power-to-Gas technology of interest is the low-temperature co-electrolysis of CO2 and H2O, which is an emerging technology for electrochemically converting them to syngas. Particularly, excess electricity available from intermittent renewable energy resources is intended to be the main energy source for the co-electrolysis. A conceptual design of the co-electrolysis process is carried out to calculate its performance data including mass balances, energy demand, and capital investment. Based on them, a temporal energy system model is developed using the TIMES (The Integrated MARKAL-EFOM System) model generator. The conclusion is that deploying the co-electrolysis process in the Korean power generation sector can reduce greenhouse gas emissions and also save the overall system cost when the syngas production cost is lower than the purchasing cost of liquid natural gas. The beneficial impacts are limited by the amount of available excess electricity and the co-firing ratio limit in the gas-fired power plants. Finally, the overpotential and current density, as uncertain parameters of the co-electrolysis process, are found to affect the syngas production cost most strongly.