Здраствуйте,
попробуйте следующее
1)HttpWebRequest hwr = (HttpWebRequest)WebRequest.Create("...");
hwr.Method = WebRequestMethods.Http.Post;
hwr.ContentType = "application/x-www-form-urlencoded";
hwr.ContentLength = request.Length;

StreamWriter SW = new StreamWriter(hwr.GetRequestStream());
SW.Write(request);
SW.Close();



2)WebClient wc = new WebClient();
string response = wc.DownloadString("...");

P.S. к 1 надо добавить еще
HttpWebResponse rspn = (HttpWebResponse)hwr.GetResponse();
StreamReader SR = new StreamReader(rspn.GetResponseStream());
response = SR.ReadToEnd();
docResponse.LoadXml(response);