Few months back I presented a paper in conference and sent the same for possible publication in the conference proceedings. I got no response for a long time and with the consent of my supervisor sent the same paper to a journal with some modifications in the paper. I want to know is it okay to send the same paper to both? As far as I feel, its not right, still I need opinions. Thanks in advance.
Apparently conference has different meanings in different disciplines. In the fields with which I am familiar, conference publications are either limited to abstracts or are published in proceedings which are just as official as a journal In the former case there is of course no problem but in the latter sending the same paper off to a conference (for proceedings publication) and to a journal would not be correct. Therefore, you need to figure out what norms apply in your field, particularly whether the conference proceedings is a real publication.
You could start from reading the journal publication agreement. It usually is very explicit on whether it is permitted or not to re-publish the results which appear in conference proceedings. In my discipline it is typically not permitted unless significant changes to the text and content have been introduced. Therefore, you should decide for yourself how significant were the modifications you made.
If the new paper is "much more" than a conference one, you could keep both, but you should tell the journal editor that this work is based on a conference proceedings which are yet in review.
If the new paper is not significantly different from the conference, you should withdraw one of them before the reviews arrive.
In any field it is unacceptable to have a paper published twice. Therefore, you should look at
Which kind of publication conference has. If it is a formal one, with a known (or even less known) publisher, ISBN, indexing - then it counts like a publication and you cannot publish it as it is in a journal. If those are abstracts proceedings, or sth printed just for distribution to the conference/workshop participants - it should be safe to submit it somewhere else.
Journal/Conference requirements - in most cases it will be specified that they look for novel papers, not published and not under submission somewhere else.
Most conference publishers (but it is more complicated in your case, as you publish proceedings of best papers in journal, not in proceedings book) would allow you to submit significantly revised/extended paper to a journal.
In your case it is probably best to contact the conference organizers and check if any formal publication planned and if your paper is selected for this publication (if it is not clear from the conference website and reviews). And do not submit to a journal before you clarify all those things - as you can get your paper retracted as double submission, with your department being informed about this...