Create Excel from SPListItemCollection and upload to a Document Library

Here is the scenario, I have a Timer Job. It sends Monthly summary of a list based on certain filters. I am able to generate Datatable from SPListItemCollection that I get using CAML. I would like to know how to generate an Excel file and upload it to a Document Library on the fly. Any suggestions?

P.S: Generating Excel from Datatable is not an issue, I am unable to figure out How to upload a Document that was just created from code (StreamWriter)?