Trying to reamp through Radial Reamp unit from my Line 6 Spider II amp. As soon as I place the mic (Shure SM57), getting noise/feedback and I am not to get the right sound. This feedback happens on stage, when mic gets close to the speakers. How do I avoid this?
 
Thanks in advance. 

I wouldn't use a mic to reamp, go direct...

How are you connecting things?  What is the order in the audio path?
If you're getting feedback it's because you're sending the mic signal back to the amp,  you shouldn't be doing that while re-amping (or ever, really!).
 
EASY way to re-amp.  (ignoring inserts)
Record guitar dry with DI direct to Sonar. You can have any plugin you like on the track while recording so you're monitoring an "amped" sound, turn it off when done.
Set the output of this track to go a spare soundcard output ONLY,  NOT the Master bus or Main audio outputs.
Connect the spare soundcard output to your amp using your preferred method, (re-amp box, etc)
Mic the amp using your preferred method as if you were recording a live guitar.
Connect the mic to a NEW track in Sonar, set the output to your 'guitars bus' (if you use one) or the Master bus.
DO NOT send it to the same output you already sent the dry track to (ie, back to the amp)
 

Simplest method is to mute the armed track (however method you use) to make sure that signal stays out of the chain to the amp. A buss allows monitoring the re-amped signal (as Karyn suggested), and can also do this via routing that track's output to a separate output for the 18i8 (to a headphone monitor); but headphones would be recommended in either case so that the microphone cannot catch that signal.

Thanks for the advice.. I will try it again. I think I screwed up on the Output part for the new track where I was bringing the amp signal in from the microphone.. also I will try to mute the armed track. Other than that, I think I followed all the steps Karyn had suggested. 
This may make my day today... can't wait to get off from work and try it out :-) (damn you 9-5 schedule :-))