Integrated consideration of carbon dioxide reduction, terminated energy conversion, and resourceful disposal of wastes like plastics broadens a new horizon for green metallurgical processes. Here, a chlorine-free plastic gasification process in situ utilizing converter slag and flue gas heat was proposed as an example, and this complicated system’s superiority was comprehensively evaluated in terms of syngas preparation efficiency and carbon dioxide reduction depth through thermodynamics. Specifically, the effects of process parameters such as temperature, pressure, and H2O/P, CO2/P, and SS/P ratios on both were discussed in detail; moreover, two sets of theoretical operation conditions were explored for the maximum syngas preparation efficiency or carbon dioxide reduction. Furthermore, the effects of waste plastics and the gasification agent amount on reaction system heat balance were discussed when the H2/CO ratio in syngas gas was 3:1 or 2:1 for CH4 or CH3OH preparation, respectively, for the purpose of terminated product quality regulation. The results showed that H2 and CO yields and the CO2 conversion rate increased as the temperature increased but decreased as the pressure increased, H2 yield was promoted by the H2O/P ratio instead of CO yield and the CO2 conversion rate, and the CO2/P ratio had a positive effect on CO yield but not on H2 yield and the CO2 conversion rate; CH4 and CaCO3 yields increased as the SS/P ratio increased. Meanwhile, chlorine-free plastic consumption increased as the temperature increased, and less gasification agent was needed. However, this system had the maximum syngas yield, namely, 44.82–64.13% H2 and 88.88–135.69% CO when temperature, pressure, and H2O/P, CO2/P, and SS/P ratios were 800–1000 °C, 1.0 atm, 1–2.0, 1.2–3.0, and 1.0, respectively; with respect to CO2 reduction efficiency, capacity ranging from 75.17 to 90% can be obtained when 800–1000 °C, 1.0 atm, 0–0.8, 0.4–1.2, and 1.0 were adopted. Definitely, the strategy of in situ converter slag and flue gas heat conversion utilizing chlorine-free plastics was indeed feasible for industrial application.