A method and apparatus to conduct O-2/CO2 combustion or oxygen-enriched combustion. The boiler exhaust gas passes through a Gas Primer Sector (GPS) combined with the boiler to effect heat transfer to the combustion gas in indirect counter-flow heat exchange. Sharply reduced gas flows result from using largely moisture-free CO2 as diluent for O-2 in the combustion gas which allows long residence time at low velocity for maximum heat transfer from the exhaust gas to the combustion gas. Most particulate drops out and most moisture is condensed from the cooled mostly CO2 exhaust gas. The larger portion is blended with oxygen for the combustion gas and reheated and returned to the boiler through the integrated GPS; the smaller portion is cleaned and separated, the CO2 released or recovered. The complete exhaust gas-combustion gas cycle may be 30 to 90 seconds and preferably about 60 seconds. The high heat capacity of CO2 allows a much higher oxygen content in the combustion gas compared to open-cycle air combustion with a large nitrogen content of lower heat capacity. Efficiency is increased. Final exhaust gas separation and recovery is simplified. Condensate is processed to boiler feedwater or portable water quality. NOx is eliminated, mercury fume captured and CO2 output reduced.