Response from WebServices SOAP Attachments

I need your help to solve a problem with a web service. I am consuming a WS with attachments. I do not have to get a valid answer and store them in a string, however when I want to pass this to XML I get an error because the response brings the following

My problem is that I can not save these in an XML document because it is not valid I need to truncate this to a valid XML ...

I leave the code as I invoke the WS

string xmlCompleto = postData.Trim();

        //Documentxml.LoadXml(xmlCompleto);

        string action = "doLogonUser";
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://webservicesejemplo");
        request.Method = "POST";
        byte[] data = encoding.GetBytes(xmlCompleto);

        //byte[] data = Encoding.UTF8.GetBytes(postData.ToString());
        var myproxy = new WebProxy();
        request.AllowWriteStreamBuffering = false;
        //request.Headers.Add("SOAPAction", action);
        request.ContentType = "text/xml; charset=utf-8";
        request.Accept = "text/xml";
        request.KeepAlive = false;
        request.ContentLength = data.Length;
        Stream dataStream = request.GetRequestStream();
        dataStream.Write(data, 0, data.Length);
        dataStream.Close();

        HttpWebResponse response = (HttpWebResponse)request.GetResponse();
        //Console.WriteLine(((HttpWebResponse)response).StatusDescription);
        dataStream = response.GetResponseStream();
        StreamReader reader = new StreamReader(dataStream);
        string responseFromServer = reader.ReadToEnd();
        //Console.WriteLine(responseFromServer);


        reader.Close();
        dataStream.Close();
        response.Close();


        return responseFromServer;

I'm desperate I can not solve this, I've searched for information but nothing yet.