Agree with the previous posters.

From CDG (TGV/RER station in Terminal 2, or RER specific station near Terminal 3) take the RER B line to Gare du Nord underground station. There connect (walking) to RER Line E, direction Hausmann St Lazare, from where you can walk to St. Lazare station. And from St Lazare, train to Rouen.

From CDG to St Lazare it is around 1 hour, from St Lazare to Rouen around 1h10 ("Corail trains").