Although, it may sound stupid to you and me for having to provide an evidence of proficeiency in English Language, even after- as you mentioned, studying in the US for several years, it is a general criteria for an official procedure in a written format. Therefore, to fulfil this firstly, I suggest you write a strong letter that can prove your proficiency in English. But remeber you will have to prove your command in all areas like writing, speaking, comprehension etc. If you have any sort of prublications in the media that have been published in English, you can attach them and that will add weight to your letter and serve your purpose. Also any recommendations form your college and professors will be positive.

 Secondly, if the Immigration officials are not satisfied from your submission (proof of english) over your command of English language, they will send you a written notification explaining that your english proficiency is not up to thier standards of evaluation for the application process. They will then, clearly give you a choice whether to calculate your points from whatever proof you have submitted for english language or to appear for IELTS and submit the result. They will give you 3-4 months of time frame to submit the IELTS results if you wish to give one.

Hope that helps.