Capturing CO2 from waste-fired combined heat and power (w-CHP) plants has attracted increasing attention. However, there has been no method that can guide the operation of CO2 capture for w-CHP plants to attain economic feasibility. To bridge this knowledge gap, this paper proposes a novel method based on the marginal cost of CO2 capture (MCoC) for the operation planning of w-CHP plants at different time scales. Two operating rules (ORs) are considered, which are based on the hourly MCoCs (OR1) and monthly MCoCs (OR2), and a real w-CHP plant is used as a case study. Results reveal that, under the electricity price of 2020 and with a carbon allowance price of 25 €/tonne, the integration of CO2 capture decreased the net revenue by 7.3 million Euro (M€) and 5.4 M€ under OR1 and OR2, respectively, compared to the reference plant without CO2 capture. Although OR2 could lead to a lower revenue loss, more CO2 could be captured under OR1. Key factors affecting CO2 capture include the electricity price, the fossil share of waste, the transport and storage cost, the price of carbon allowances, and the price of waste. Higher electricity prices can benefit the w-CHP plant under OR1, while increases in the fossil share of waste, transport and storage costs, and prices of waste decrease the plant's revenue under both ORs. To achieve net revenue from CO2 capture, the price of carbon allowances must exceed the thresholds of 58 €/tonne and 49 €/tonne under OR1 and OR2, respectively.