Captured CO2 from CO2 emission sources contains impurities such as N-2, CH4, H2O, O-2 and Ar, and these are transported with the CO2 in pipelines under supercritical condition. The aim of this study is to experimentally investigate the effects of impurities in CO2 + N-2 and CO2 + CH4 on the in-tube heat transfer and pressure drop under seawater environmental conditions during pipeline transportation. The experimental apparatus consisted of a test section, heat exchangers that were connected to two chillers, and a magnetic gear pump. The test section was made by a cooper tube that was inserted into a Polyvinyl Chloride (PVC) pipe to form a double-tube. The operational temperature and pressure of the CO2 mixture ranged 25-55 degrees C and 80-100 bar, respectively. The mass flux was varied by 300, 500, and 700 kg m(-2) s(-1). The mole fraction of CO2 in the CO2 mixtures was varied from (1.00-0.95). When the CO2 mole fraction decreased from 1.00 to 0.95, the maximum heat transfer coefficients of CO2 + N-2 and CO2 + CH4 decreased by 4157 and 1224 W m(-2) K-1, respectively, and the average pressure drop of CO2 + CH4 increased by 29.87%. (C) 2019 Elsevier Ltd. All rights reserved.