If you do not have (much) luggage you can take the RER Line B from CDG to Chatelet and change to RER Line A (direction La Defense) and get off at Auber which is the next stop. Auber is connected to the Metro station Opera and is right in front of the Grand IC.
I have stayed at the Grand IC and enjoyed it (despite having my IC hotel jinx happening at every IC hotel I have stayed at; perhaps its me?) -- ask for a room with an Opera view. Stunning at night, and in the early morning if the sun is shining. The Head Concierge, Gilles, is friendly and very competent.
The Air France Bus -- tickets are available in the air terminals or from the bus drivers. Tel: 01 41 56 89 00 or
www.cars-airfrance.com