Integrating carbon dioxide (CO2) capture in biomass or waste-fired combined heat and power (CHP) plants has been considered a key measure to achieve negative emissions. To support decision-making, an accurate assessment of the potential contribution and the associated cost from the national perspective is urgently needed. This paper proposed a bottom-up approach based on a dynamic modelling to evaluate the potental of nationwide negative emissions. As heat supply is often prioritized by CHP plants, unchanged heat generation is a prerequisite of this study. Two operating modes (OMs) for the integration of CO2 capture are investigated, which can represent the upper and lower boundaries of CO2 capture: OM1 aims to maximize the amount of captured CO2, while electricity generation can be sacrificed; OM2 aims to maximize the amount of captured CO2, while the electricity generation is maintained unchanged. Sweden is employed as a case study. Results show that operating CO2 capture in OM1 can achieve 8.7 million ton CO2 nationwide negative emissions a year, while operating CO2 capture in OM2 can generate 4.3 million ton CO2 positive emissions a year, which represents a reduction of 6.3 million tonCO2 a year compared with the reference plant without CO2 capture. The levelized costs of CO2 avoided are 36.9 USD/tonCO2 and 52.0 USD/tonCO2 for OM1 and OM2, respectively. The biogenic fraction of waste has a significant influence on negative emissions. According to the Swedish climate goal about bioenergy with CO2 capture and storage (BECCS), to achieve 3 million ton negative CO2 emissions a year, the minimum biogenic fractions should be 32.8% and 84.3% for operating CO2 capture in OM1 and OM2, respectively; in contrast, to achieve 10 million ton negative emissions a year, biomass and waste-fired CHP plants have to operate CO2 capture in OM1 and the biogenic fraction needs to be over 59.9%.