It is!
All the info about the bus to the airport http://www.stm.info/en/info/networks/bus/shuttle/more-about-747-aeroport-p-e-trudeau-centre-ville-shuttle
However, for anyone who wants to wander around a bit, regular buses and metro (underground train) is more practical. The metro is linked to many building by indoor tunnels, could be practical if the weather stays as cold as now...