The sprocket wheel on my Shimano Nexus 7(SG-R46) no longer rotates freely when free-wheeling my bike
OK. I love the nexus series. They are great pieces of kit with a lot of utility in crappy environments. Low maintenance, ease of use and nearly bullet proof. 

Except when it comes to water. Your model uses grease to lubricate the internals, not oil. Water intrusion, even in small amounts, will cause the symptoms you describe. The hub most likely needs to be taken apart, cleaned and repacked with the super special shimano grease (I wouldn't play with that one here as the wrong grease might cause damage.) It's pretty straightforward if you take your time.  For more information, including diagrams, instructions and shimano factory manuals go to Sheldon Brown's (RIP) at Nexus Internal Gear