An exhaust aftertreatment system for a compression-ignition internal combustion engine , the exhaust aftertreatment system comprising a base exhaust aftertreatment system and a modular exhaust subsystem for purifying an exhaust gas feedstream of the compression-ignition internal combustion engine upstream of the base exhaust aftertreatment system , wherein the modular exhaust subsystem is a stand-alone modular system configured for being inserted between the internal combustion engine and the base exhaust aftertreatment system , the modular exhaust subsystem comprising: a selective catalytic reduction (SCR) catalyst , a first exhaust gas sensor and a first temperature sensor arranged to monitor the SCR catalyst ; a reductant delivery system arranged to inject a reductant into the exhaust gas feedstream upstream of the SCR catalyst ; a controller , operatively connected to the reductant delivery system and in communication with the first exhaust gas sensor and the first temperature sensor , the controller being further in communication with a second exhaust gas sensor and a second temperature sensor that are arranged to monitor the base exhaust aftertreatment system ; and the controller being further in communication with an engine-out exhaust gas sensor arranged to monitor the exhaust gas feedstream upstream of the SCR catalyst , wherein the controller includes an instruction set that is executable to control the reductant delivery system to inject the reductant into the exhaust gas feedstream upstream of the SCR catalyst based upon inputs from the engine-out exhaust gas sensor , the first and second exhaust gas sensors and the first and second temperature sensors , wherein the instruction set is executable to control the reductant delivery system to inject the reductant into the exhaust gas feedstream upstream of the SCR catalyst to achieve a target reductant/NOx ratio upstream of the base exhaust aftertreatment system . a selective catalytic reduction (SCR) catalyst , a first exhaust gas sensor and a first temperature sensor arranged to monitor the SCR catalyst ; a reductant delivery system arranged to inject a reductant into the exhaust gas feedstream upstream of the SCR catalyst ; a controller , operatively connected to the reductant delivery system and in communication with the first exhaust gas sensor and the first temperature sensor , the controller being further in communication with a second exhaust gas sensor and a second temperature sensor that are arranged to monitor the base exhaust aftertreatment system ; and the controller being further in communication with an engine-out exhaust gas sensor arranged to monitor the exhaust gas feedstream upstream of the SCR catalyst , wherein the controller includes an instruction set that is executable to control the reductant delivery system to inject the reductant into the exhaust gas feedstream upstream of the SCR catalyst based upon inputs from the engine-out exhaust gas sensor , the first and second exhaust gas sensors and the first and second temperature sensors , wherein the instruction set is executable to control the reductant delivery system to inject the reductant into the exhaust gas feedstream upstream of the SCR catalyst to achieve a target reductant/NOx ratio upstream of the base exhaust aftertreatment system . <EOS>