 If you are found to be eligible to take part in this study, you will begin taking imatinib mesylate. Depending on when you begin treatment on this study, you may be asked to take 3, 4, or 6 tablets of imatinib mesylate by mouth once a day. Your doctor will also ask you to take folic acid tablets or a multivitamin with folic acid during the week before you receive your first infusion of pemetrexed and then every day while you are on this study. You will also be given an injection of vitamin B12. The vitamin B12 shot will be repeated every 9 weeks during treatment on this study. Folic acid and vitamin B12 will help to decrease the risk of severe side effects from pemetrexed. 