Comments = new Mongo.Collection('comments');

Meteor.methods({
  commentInsert: function(commentAttributes) {
    check(this.userId, String);
    check(commentAttributes, {
      postId: String,
      body: String
    });

    var user = Meteor.user();
    var post = Posts.findOne(commentAttributes.postId);

    if (!post) {
      throw new Meteor.Error('invalid-comment', 'You must comment on a post');
    }

    comment = _.extend(commentAttributes, {
      userId: user._id,
      author: user.username,
      submitted: new Date()
    });

    Posts.update(comment.postId, { $inc: { commentsCount: 1 }});
    comment._id = Comments.insert(comment);
    createCommentNotification(comment);

    return Comments._id;
  }
});

Comments = new Mongo.Collection('comments');

Meteor.methods({
  commentInsert: function(commentAttributes) {
    check(this.userId, String);
    check(commentAttributes, {
      postId: String,
      body: String
    });

    var user = Meteor.user();
    var post = Posts.findOne(commentAttributes.postId);

    if (!post) {
      throw new Meteor.Error('invalid-comment', 'You must comment on a post');
    }

    comment = _.extend(commentAttributes, {
      userId: user._id,
      author: user.username,
      submitted: new Date()
    });

    Posts.update(comment.postId, { $inc: { commentsCount: 1 }});

    return Comments.insert(comment);
  }
});

--- 
+++ 
@@ -22,9 +22,7 @@
     });
 
     Posts.update(comment.postId, { $inc: { commentsCount: 1 }});
-    comment._id = Comments.insert(comment);
-    createCommentNotification(comment);
 
-    return Comments._id;
+    return Comments.insert(comment);
   }
 });
Modify commentInsert to add notifications after commenting
