This article provides results of the in situ monitoring of carbon dioxide (CO2) removal under high pressure. An aqueous solution of methyldiethanolamine (MDEA) promoted by piperazine (PZ) for absorption rate acceleration was used. This system is promising for natural gas purification. A predictive statistical model was built using the chemometrics method and measurements of density, pH, conductivity, sound velocity, refractive index, and Near Infra-Red (NIR) spectroscopy. The CO2 capture rate ranged from 60% up to 96% at pressures in the absorber column ranging between 15 and 20 bar. The liquid stream composition was monitored at the low pressure part of a pileline at a location before a compressor stage. The concentrations of MDEA, PZ, and CO2 were predicted in-line using different sets of the measurement devices during 3 days of the measurement campaign. The developed approach allowed for prediction of the concentrations with accuracies of 0.7% for MDEA, 0.4% for PZ, and 2.5% for CO2.