If my I-751 application is in process and I'm planning to travel outside the US, will be able to re-enter the US in case my I-751 rejected while I'm gone?

That will depend. If you have your valid passport, expired card and I-751 receipt, you should be readmitted. However, if the airport searches the database and determines it was denied, you may be denied. You should check on when you filed and how long it takes at your service center to process to make sure your case is not going to be denied while you are gone. I would be happy to discuss my opinion, but we need to know what you provided and why you think it will be denied.